This invention relates to an exhaust gas purifier provided in the exhaust line of an engine and including a trapper which can be regenerated with increased efficiency.
Exhaust gas produced by a diesel engine contains particulates which are mainly carbon. Such particulate matter (hereinafter called PM) is a major cause of air pollution. To remove PM, various means have been developed.
One such conventional exhaust purifier that is now practically used includes a trapper made of a porous ceramic material, and an electric heater for burning PM trapped on the trapper to regenerate the trapper, i.e. to improve the flow of exhaust through the trapper. Lately developed trappers include cylindrical metallic porous members.
These trappers are actually filters which trap PM mainly on their surfaces at the exhaust incoming side.
Among such exhaust gas purifiers is one with electric heaters for regenerating the trapper provided opposite the front face of the trapper (unexamined Japanese patent publication 3-258911). Unexamined Japanese patent publication 2-27107 proposes to burn trapped PM by producing arc discharge between the trapper and electrodes (or conductors) inserted in deep exhaust passages formed in the trapper. It has been believed that these devices can burn trapped PM efficiently because PM is trapped mostly on the front side of the trapper. But we have found out that some part of PM penetrates deep into the trapper, and such PM is difficult to burn during regeneration.
Further, the device of unexamined Japanese patent publication 2-27107 has the possibility of arc discharge occurring unevenly in the inlet passage, which in turn increases the possibility of uneven burning of trapped PM. Another conventional device in which the electrodes of the above publication are replaced with electric heaters has a problem in that the heaters can short-circuit through trapped PM. Short-circuiting increases the possibility of uneven burning of PM.
In order to solve these problems, unexamined Japanese publication 5-79317 proposes to provide a first exhaust passage in the center of a third passage through which exhaust gas that has passed through the filter flows. The first passage is adapted to be opened during regeneration. When the first passage is opened, flame propagation is promoted due to eject effect. This improves the regeneration efficiency.
The device of unexamined Japanese utility model publication 4-42214 has a first electric heater provided at upstream side of the trapper and a second electric heater provided at its downstream side. The second heater burns any PM that has not been burned by the first heater.
Unexamined Japanese patent publication 5-79317 aims to regenerate the trapper efficiently while limiting the power consumption of the heaters to a minimum. In this arrangement, the exhaust passage has a double structure. A throttle valve is needed to open and close the inner first exhaust passage. Thus, the device is complicated in structure. Since the heater is provided in close proximity to the inner surface of the trapper, the heaters may be heated excessively while burning PM.
The device of unexamined Japanese utility model publication 4-42214 needs two heaters. This device is not suitable for use in a car because the capacity of the car battery is not enough to energize two heaters.
An object of this invention is to solve these problems and provide an exhaust gas purifier which can regenerate the trapper efficiently.